Geothermal energy is an important potential energy source. Past utilization of the potential energy available from geothermal reservoirs has been limited to natural hydrothermal underground reservoirs. Hydrothermal reservoirs contain sufficient amounts of indigenous fluid which may be brought to the surface, via a set of drilled wells, for the production of electrical energy such that they may be considered economically viable energy sources. Previously the energy contained in known hot dry rock (HDR) formations has gone untapped because such formations lack sufficient quantities of naturally contained fluid that may be brought to the surface to make electrical production economical.
Until recently, the exploitation of HDR reservoirs has not occurred, partly because of the difficulty and cost of drilling into hard, hot, crystalline rock, but primarily because the low thermal conductivities of such formations made it appear that heat energy could not be extracted at a useful rate, absent some type of downhole structure having a very large surface area. It was assumed that a heat transfer surface of the required area could not be created downhole by existing methods.
U.S. Pat. No. 3,786,858 disclosed that a heat transfer surface of practical size could be created in HDR formation by the common oil-field technique of hydraulic fracturing. This patent describes a method of creating a downhole structure for the extraction of heat energy from a HDR formation comprising, drilling a first (injection) well to the formation depth, hydraulically fracturing the formation from the injection well (to create a thin vertical disc having a large heat transfer surface), drilling a second (withdrawal) well to intersect the fracture, and circulating a heat exchange fluid within the fracture via the injection well and withdrawing it to the surface via the withdrawal well for the extraction of heat energy.
The process of U.S. Pat. No. 3,786,858 has several disadvantages. The first being that it utilizes only a single fracture plane to comprise its reservoir, thus limiting the rate at which heat may be extracted. Second, the withdrawal well must be precisely drilled to intersect the narrow fracture plane without intersecting it at a point that would short-circuit any substantial proportion of the created heat transfer area. Both intersecting the fracture plane and intersecting it at a precise point may necessitate fairly difficult and expensive directional drilling techniques.
U.S. Pat. Nos. 3,878,884 and 3,863,709 disclose methods for creating a geothermal reservoir comprising a plurality of fracture planes, thus greatly increasing the effective amount of surface area available for heat extraction. In U.S. Pat. No. 3,878,884, a first well is vertically drilled to the HDR formation, then deviated from vertical in a compass direction corresponding to the formation's lines of least principal stress. Subsequently, a plurality of parallel fracture planes are created in the formation by hydraulic fracturing at spaced apart positions along the deviated well. A withdrawal well is subsequently drilled above and parallel to the first well to intersect a majority of the fracture planes. In U.S. Pat. No. 3,863,709 a pair of wells are drilled and deviated from vertical through the HDR formation. A series of fracture planes are hydraulically induced from one of the wells and propagated through the formation to intersect the second well. Thereafter, the area along the second well at which intersection occurs is located by injection of a radioactive tracer into the fracture plane from the first well. The second well is perforated at this point to place it in hydraulic communication with the other well.
In both cases the disadvantages of properly locating the second well within the fracture complex still pertain. In U.S. Pat. No. 3,878,884 the second well must be drilled to intersect the fracture complex without short-circuiting any substantial proportion of the effective heat transfer surface of the fractures created. In U.S. Pat. No. 3,863,709 the second well must be drilled parallel to and within a preset distance of the first well, the maximum distance being the maximum radius along which a singularly induced fracture may be expected to propagate. Thus, the possibilities exist that intersection may not occur if this distance is too large or that effective heat transfer surface may be short-circuited if this distance is too small.